Method and devices for controlling loads connected to a power line

ABSTRACT

The present invention is related to a method and devices for controlling at least one load connected to an AC power line. The method is based on the introduction of interruptions in the line voltage, which correspond to data being sent. The method centers on interruptions of variable length, yielding the opportunity of sending more data with a reduced number of interruptions. The invention is further related to a particular design of the transmitter and receiver. The transmitter is equipped with a means for measuring the line current. The receiver is equipped with a comparator with a feedback connection. The receiver is also capable of increasing momentarily the line current, which creates the possibility of sending information back towards the transmitter.

This Application is a National Phase Application filed under 35 U.S.C.371 claiming the benefit of International Application No. PCT/BE02/00107filed Jun. 26, 2002, which has priority based on European Patent Office(EPO) Application No. 01870146.6 filed Jun. 28, 2001.

FIELD OF THE INVENTION

The present invention is related to a method and to devices forcontrolling loads such as lamps connected to a power line. Inparticular, the invention is related to a new type of power line modem,both for sending and receiving data, allowing to perform such a control.

STATE OF THE ART

The control of loads connected to a power line concerns the turningon/off of loads, monitoring their correct functioning, controllingspecific features such as speed in case of motors, or light intensity incase of lamps.

Document U.S. Pat. No. 4,797,599 describes a circuit for lamp dimmingwherein a control signal is sent over a separate wire. The main drawbackof these techniques lies in the necessity of this separate wire, whichcomplicates the design.

A powers line modem for lamp dimming is mentioned in the documententitled ‘A novel dimmable electronic ballast for street lighting withHPS lamps’, P. Van Tichelen et Al, conference record of the 2000 IEEEIAS conference, October 2000. These powerline modems add a modulatedvoltage signal (e.g. between 9–90 kHz) on the power line, so without theneed for a separate wire. They are however relatively expensive toimplement and susceptible to network noise. They also have a high powersupply rating.

Moreover, this type of modem requires the use of repeaters to sendinformation over long distances. These modems are not universallycompliant with international standards, the high frequencies mentionedbeing different according to different standards.

Document WO-A-9206552 is related to a transmitter and receiver of dataon a power line, by using the momentary interruption of the line voltageat or near the voltage zero crossing. A transmitter comprising a switch,in particular a triac is used to create these interruptions on the linevoltage. As acknowledged in the document itself, this gives rise toproblems when a capacitive or inductive load is used. To solve thisproblem, the cited document suggests an alternative transmitter andreceiver circuit involving analysis of the voltage waveform with amicro-controller. This is however a complicated solution.

In the receiver described in WO-A-9206552 (FIG. 4), the presence of avoltage interruption is detected by a comparator 101, which produces asquare wave at double the frequency of the supply voltage. The presenceof a notch in the signal is detected by measuring the length of thepulses, delivered by 101.

AIMS OF THE INVENTION

The present invention aims to propose a method and devices enabling thecontrol of one or more loads connected to a power line, in a simplifiedand cheaper way compared to prior art methods and devices.

The present invention further aims to propose a set of devicesexhibiting a higher immunity to network noise, and a low harmonicdistortion, while necessitating a lower power supply rating.

Finally, the present invention aims to propose a method and devices,which allow to avoid the use of repeaters, and which are compliant withall international standards.

SUMMARY OF THE INVENTION

The present invention is related to a device for receiving data on an ACpower line which is connected to a power line voltage, and which istransporting a power line current through a set of two conductors, saiddevice being characterised by the fact that it comprises:

-   -   a microcontroller,    -   a comparator, whose two inputs are connected through resistors        to said two conductors, one input being connected to one        conductor, the other input to the other conductor, so that said        comparator produces at its output a square wave with the same        frequency of said power line voltage,        wherein said microcontroller comprises circuitry to measure the        duty cycle of each period of said square wave produced by said        comparator, and circuitry for deriving from said duty cycles an        amount of data, and circuitry for sending a command signal        towards a load, and wherein a feedback connection comprising a        resistor is present between the output of said comparator and        one of the inputs of said comparator.

According to the preferred embodiment, the device for receiving dataaccording to the invention further comprises:

-   -   a transistor and one or two diodes, each diode's anode being        connected to a conductor of said power line,    -   a resistor connected between the cathode(s) of said diode(s) and        the collector of said transistor, said transistor being turned        on or off by said microcontroller, so that the line current of        said power line is increased upon switching on of said        transistor.        In the receiving device according to the invention, one of the        terminals of the supply voltage of said comparator is connected        to one of the conductors of said power line, through at least        two diodes.

The invention is equally related to a device (1) for sending data on anAC power line, which is connected to a power line voltage, and which istransporting a power line current through a set of two conductors, saiddata being transmitted towards a receiver according to the invention,said device comprising:

-   -   a switching device, placed in one of the conductors of said        power line,    -   a microcontroller,    -   a first connection between said microcontroller and said        switching device,    -   a second connection comprising one or more conductors between        said microcontroller and at least one of said conductors of said        power line,        wherein said microcontroller comprises circuitry for producing a        signal for changing the condition of said switching device        through said first connection, circuitry for measuring an        electric value through said second connection, and circuitry for        introducing a predefined time delay before changing the        condition of said switching device, characterised in that said        device further comprises means for measuring the line current.

According to the preferred embodiment, said means for measuring the linecurrent comprises a shunt resistor in one of the conductors of saidpower line and two connections between said microcontroller and pointsrespectively before and after said shunt resistor.

According to one embodiment, said switching device is a triac or a groupof two thyristors, said triac or said two thyristors being preferablyequipped with a snubber network.

According to another embodiment, said switching device is a transistor,with the further addition of a diode bridge.

According to the preferred embodiment, the transmitting device of theinvention, further comprises a connection (14) between saidmicrocontroller and a point located between said switching device andthe loads controlled by said device.

The transmitting device of the invention may further comprise a meansfor activating a dummy load circuit. Said dummy load circuit maycomprise a transistor, a diode bridge, and a resistor. Said means foractivating said dummy circuit comprise an optocoupler. Said dummy loadcircuit may be incorporated into said device for sending data.

The invention is equally related to the use of a device for receivingdata according to claim 1, for commanding the switching on or off of adummy load, said dummy load comprising a resistor, as well as to the useof a device for receiving data according to claim 2, for commanding theswitching on or off of a dummy load, wherein said resistor is used asthe dummy load.

The present invention is also related to a method for controlling atleast one load connected to an AC power line, which is connected to apower line voltage, and which is transporting a power line currentthrough a set of two conductors, said controlling taking place throughthe sending of data, said data being sent by momentarily interruptingthe line voltage, characterised in that said method comprises thefollowing steps:

-   -   interrupting the line voltage of said AC power line during        predefined interruption intervals,    -   detecting said interruption intervals by detecting a deviation        of the duty cycle of said line voltage from the duty cycle of        the non-interrupted line voltage,    -   deriving a message from the number and the length of these        interruption intervals.

According to the preferred embodiment, the method of the inventioncomprises the steps of:

-   -   introducing a first interruption, called reference interruption,    -   measuring the deviation of the duty cycle from the duty cycle of        the non-interrupted line voltage, and defining said deviation as        a reference value,    -   introducing interruptions during one or more cycles of the line        voltage, following said reference interruption,    -   measuring the deviation from the non-interrupted duty cycle,        caused by said interruptions following the reference        interruption, and comparing said measured deviations to said        reference value,    -   deriving from said comparison a set of transmitted data.

According to the preferred embodiment, after a fixed number of cycles,the reference interruption is introduced again in the line voltage, anda data transmission packet which was sent in between said first andsecond reference interruptions is valid when the duty cycle deviationcaused by this second reference interruption is sufficiently close tothe duty cycle deviation caused by said first reference interruption.

In the method of the invention, one interruption preferably consists oftwo interruptions of the same length, and introduced on two consecutiveflanks of the voltage, i.e. either a rising and descending flank, orvice versa.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the method of sending data by interrupting the linevoltage.

FIG. 2 a and 2 b represent two embodiments of the circuit of atransmitter according to the invention.

FIG. 3 represents the circuit of a receiver according to the invention.

FIG. 4 represents a receiver according to the invention connected to adummy load.

FIG. 5 represents the circuit of a dummy load, which can be activated bythe transmitter of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is related to devices and one particular methodfor controlling one or more loads connected to an AC power line, forexample one or more lamps which are to be switched on/off or dimmedindependently of each other. The basic embodiment of the method which isused here is known in the art. It is based on the technique of phasecontrol of the line voltage in order to send data, for example asequence of 0 and 1, towards one or more loads, said data beingtranslated into commands, e.g. ‘switch off a motor’, ‘dim a lamp’,etc. .. . . The phase control is performed by short interruptions of the linevoltage, preferably starting shortly before or after or simultaneouslywith a zero crossing of said line voltage. The length of theseinterruptions are short compared to the period of the line voltage,preferably not longer than 9% of said period. This means 1.8 ms for a 50Hz signal. The subsequent fluctuations in the wave form of the linevoltage are detected by a suitable device connected to the load, andtranslated into command signals. In this way, the digital information isnot sent through a separate cable, but by a slight modification of theline voltage itself. This modification can be kept sufficiently reducedin order to minimise harmonic current distortion.

One embodiment of the method is illustrated in FIG. 1, which shows thesinusoidal line voltage 100 and line current 101 on a power line, inthis case at 50 Hz. The case shown is relevant to a purely resistiveload, i.e. voltage and current are in phase. At a zero crossing of thecurrent and voltage, an interruption or ‘hack’ 102 is caused of about1.5 ms in length, which is repeated by a second hack 103 of the samelength on the following flank.

The drawing indicates the translation of this particular sequence ofhacks into digital information (0 or 1). According to this embodiment, a‘hack’ on two consecutive flanks represents a ‘1’. The absence of a hackon two consecutive flanks after this first ‘1’,represents a ‘0’. Themeaning of the interruptions may be reversed: two ‘hacks’ are thenequivalent to ‘0’ and two continuous zero crossings are equivalent to‘1’.

In all cases, a sequence begins with a code that starts with two hacks,to indicate that a message is about to be sent. This first code may thenbe followed by an identification code for the specific load or loadsaddressed, and finally by a command code with a fixed length.Preferably, at the end of a message, an indication, for example twoadditional ‘hacks’ are added to mark the transition from sending of datato normal line voltage supply without hacks. For example, a message mayconsist of the following 4 parts:

-   -   start bit (2 hacks)    -   8 address bits    -   8 command bits    -   end bit (2 hacks)

In the receiver of the invention, described later, the detection of thecommand signals occurs through a comparator, whose output is a blockwave 104, wherein the length of the blocks is representative of thevalue of the bits that are transmitted. A counter device is used todetect these differences in length. In the embodiment shown, the blockwave 104 is ‘high’ during the negative half cycle of the voltage, whennormal operation occurs (no interruptions). The output remains highduring the interruptions, which causes the block length to becomelarger, when two consecutive interruptions are given on two consecutiveflanks. Means must be provided to make sure that the output does in factremain high during hacks, as will be explained further in the text.

In the previous embodiment, one voltage interruption on a falling edgeand one interruption on the subsequent leading edge represents one bit.The first interruption can equally be on a leading edge.

The present invention is related to a method for sending more thenone-bit information by one voltage interruption, by making the length ofthe interruption variable.

The preferred embodiment of the method of the invention comprises thefollowing steps:

-   Step 1: A reference interruption is introduced. This interruption    has a predefined length, and is introduced for example on two    subsequent line voltage flanks, as described above.-   Step 2: the reference interruption causes a deviation from the    normal 50% duty cycle of the non-interrupted line voltage. This    deviation is now measured and defined as a reference value.-   Step 3: during one or more of the following line voltage cycles,    interruptions of varying length are introduced. In every case, an    interruption of the same length is introduced on two subsequent    flanks, as described above. However, the length of a pair of    interruptions in one line voltage cycle may be different from the    length of the pair of interruptions introduced during the following    line voltage cycle.-   Step 4: the interruptions are detected as deviations from the 50%    duty cycle, deviations which are now compared to the reference    value.-   Step 5: the comparison yields a set of different values, for example    a set of ratios of measured deviation values, relative to the    reference value. Other ways of comparing may be used. The length of    the hacks may be measured and substracted from the length of the    reference hacks.-   Step 6: these values are translated, according to a predefined    scheme, into a set of transmitted data, This method allows to    transmit more data by a lower number of interruptions, which allows    a quicker and more efficient control of loads connected to a power    line.

In the preferred embodiment, an additional step is added, whereby, aftera set of data has been transmitted in the way described above, thereference interruption is introduced once again, on two subsequentflanks. The duty cycle deviation is once again measured and should bethe same as the original reference value. This is a way of checkingwhether the line load condition has changed during the transmission,thereby checking the validity of the transmission itself.

The line voltage and current of FIG. 1 are in phase, which is only truefor a resistive load. In reality, a load will always have a degree ofcapacitive or inductive impedance. As will be explained later, the phasedifference between V and I must remain between limits, in order for thetransmitter and receiver of the invention, when equipped with a triac,to work properly. It could occur that a ‘hack’ is not introduced or istoo short in length, due to an excessive phase difference.

Fluctuations in capacitive or inductive content may occur, causingsudden changes of the interruption lengths, which would yieldtransmission errors. Such fluctuations can be found by surveying thedifference in the duty cycle deviation between a first referenceinterruption and a second reference interruption, introducedrespectively before and after the sending of a data packet according tothe above described method. The second reference interruption allows tocheck whether the duty cycle deviation has remained stable during thetransmission of data. It is assumed that the line load conditions duringthe transmission of one data packet do not change considerably.Therefore, if the deviation has not changed beyond a predefinedtolerance limit, it can be concluded that the transmission has been sentin a correct way.

The present invention is further related to a combination of two devicesneeded in order to send and receive data by line voltage interruptions:a ‘cluster’ power line modem (PLM), the transmitter, used to control oneor more of a specific group of loads, and a load PLM (receiver), inconnection with each load. The transmitter comprises at least amicrocontroller and a device able to interrupt the voltage, such as atransistor or a triac, or more generally two thyristors inanti-parallel, not necessarily packed into the same unit, as is the casefor a triac. FIG. 2 a shows an embodiment, based on a triac. FIG. 2 bshows an embodiment that uses a transistor.

The transmitter is itself operated from a central control station 4, andcontrols a number of loads 2. FIG. 2 further shows the conductors 5 and6 of the power line, an AC supply voltage being present between theinput terminals 7 and 8.

The transmitter in FIG. 2 a comprises first of all a triac 10, equippedwith a classic snubber network, consisting of a capacity Cs and aresistor Rs, in parallel with the triac. The triac 10 is placed in thefirst conductor 5 of the power line and fired through connections 11 and15, preferably through a triac driver 9, connecting the triac to amicrocontroller 12, which is in turn operated by the control station 4.The microcontroller 12 is supplied by a voltage VDD (for example 5V DC)referenced to a reference voltage Vss.

The triac 10 is switched off as soon as the line current becomes zero.The zero crossing of the voltage is detected through connections 13 and22. Under normal circumstances, meaning when no message is sent, thetriac 10 is continuously fired through connections 11 and 15, so thatthe triac is immediately re-fired after a zero crossing of the currentand no interruption of the line current and voltage takes place.However, when an interruption needs to be introduced, the firing of thetriac after zero crossing of the current is slightly delayed, over aperiod of time (for example 1.5 ms for a 50 Hz signal) programmed intothe microcontroller, and starting from the moment when the voltagebecomes zero. This period of time may be constant or variable. In thelatter case, the transmitter may be used to implement the method of theinvention, described above. The delay causes a detectable interruptionof the line voltage, leading to a line voltage curve 100 such as shownin FIG. 1. The micro-controller 12 comprises programmable countercircuitry known in the art to create the delay before firing the triac10.

When using a triac as the switching device, the phase difference betweenline voltage and line current is an important factor. In the theoreticcase of a zero phase difference, the zero crossings of voltage andcurrent are simultaneous and no problem occurs (see FIG. 1). However,when the voltage zero crossing precedes the current zero crossing (i.e.for an inductive load), a very short voltage interruption length mightoccur or the ‘hack’ might be missed altogether. Similar problems arisein the case of a capacitive load.

It is to be noted that the voltage measurement through connections 13and 22 to detect zero voltage, may be replaced by a current measurement.In this case, the delay would start from a zero crossing of the current.Also, one connection (13 for example) may be present, while the otherconductor of the power line is connected to a suitable reference voltage(e.g. Vss).

The transmitter in FIG. 2 comprises an additional connection 14 from apoint in the first conductor 5, after the triac (i.e. between the triacand the loads 2), to the microcontroller 12. This allows a control ofthe transmitted signal. The microcontroller 12 may check in this waywhether all interruptions that were ordered, were effectivelytransmitted in the form of voltage interruptions. For example in thecase of inductive loads, this may not be the case, as explainedpreviously. Suitable circuitry should be added to the microcontroller 12to measure the voltage through connection 14 and to perform a correctiveaction on the transmitted signal in case a fault is detected (forexample, increasing the interruption time).

Despite the use of these corrective measures, when a triac is used, thephase difference between voltage and current should be as small aspossible, i.e. the power factor of the loads should approach 1.Therefore, the device of FIG. 2 a is ideally used in combination with aPower Factor Corrector device, for example placed before each one of anarray of electronic lamp loads connected to the power line.

Transistors can be switched independently of the line current. FIG. 2 bshows the circuit of a transmitter of the invention, wherein the triacis replaced by a transistor 17 and a diode bridge D1 to D4. Thetransistor can be switched on or off at any time through connections 18and 19, independent of the line current zero crossing. The diode bridgeis necessary to allow the current to change direction during one period.The other elements of the circuit in FIG. 2 b are identical and carrythe same reference number than the elements in FIG. 2 a. The embodimentof FIG. 2 b is an alternative to the one with the triac, but can only beused with loads which are not inductive.

In the embodiments of FIG. 2, the transmitter further comprises aresistor 20 in the second conductor 6 and a connection 21 to themicrocontroller 12. The resistor 20 is a shunt resistor, and is used tomeasure the line current. In this case, the microcontroller comprisescircuitry to be able to measure this line current, through connections21 and 22. According to the invention, a modification of the linecurrent, may be induced by one of the receivers, communicating in thisway information concerning the state of a load to the transmitter. Thisis explained in more detail after this. Another device than a shuntresistor may be used for measuring the line current, for example acurrent transformer.

The transmitter can be commanded from the control station 4 through aconnection 16. The transmitter may also be controlled by a remotecontrol through a wireless connection.

FIG. 3 shows the circuit of the receiver, also called ‘load PLM’ 3according to a preferred embodiment. Also in FIG. 3 is an example of aload 2 (e.g. a lamp with a dimming circuit). The load is connected tothe power line, via an EMI (Electro Magnetic Interference) filter 30, arectifier 31 essentially for creating a DC bus towards the load 2, aPower Factor Corrector 32, and a capacity C2 for compensatingfluctuations over the DC bus. These elements are included by way ofexample and are in no way restrictive to the scope of the invention.

The receiver 3, as shown in FIG. 3, comprises a comparator 34, amicrocontroller 35, a transistor 36. A feedback connection 50 comprisinga resistor R5 is connected between the comparator's output and one ofits inputs. The second conductor 6 of the power line is connected to theinverting input of the comparator 34 by way of a resistor R3, and saidinverting input is connected by way of a resistor R4, to the samereference potential (Vss) as the negative output of the rectifier 31.The first conductor 5 of the power line is connected to thenon-inverting input of the comparator 34 by way of a resistor R1 andsaid non-inverting input is connected, by way of a resistor R2, to thesame reference potential (Vss). The resistors R1, R2, R3 and R4 arearranged as voltage dividers, in order to produce suitable signals atthe inputs of the comparator 34. In this particular case, R5 isespecially important, since it creates the certainty that the output ofthe comparator remains high during the delay time (hacks 102 and 103,FIG. 1).

The feedback via R5 may equally take place to the inverting input of thecomparator 34. The importance of the feedback resistor is now explainedin more detail. The receiver circuit of the invention works with asingle comparator 34. This circuit can produce a square wave pulse whichis synchrone with the line frequency and which has an almost 50% dutycycle under normal operation. When a line voltage interruption isintroduced, a single comparator would not be able to detect a deviationfrom the square wave pulse under normal operation because both inputs ofthe comparator are equal and the output state would not be properlydefined. This is why the feedback resistor R5 is introduced. When theline voltage is interrupted, the resistor R5 forces the output of thecomparator 34 to remain stable, i.e. a positive output voltage if thefeed back is connected to the non-inverting input of the comparator 34or a low (zero or negative) output if the feed back is connected to theinverting input of the comparator 34.

This effect ensures the desired duty cycle shift of the comparator'soutput pulse (such as illustrated in FIG. 1), when a voltageinterruption has been introduced.

For a proper functioning of the comparator circuit, the supply voltageof the comparator 34 (VDD&VSS) is preferably defined with respect to theline voltage. A direct connection to one of the line voltages (5 or 6)of VDD or VSS of comparator 34 is not possible, this can be derived by aperson skilled in the art and was confirmed experimentally. To solvethis problem, one of the comparator supply voltages (VSS or VDD) isconnected to the output of a diode rectifier bridge 31, see FIG. 3. Ingeneral terms, it is necessary that VDD or Vss be connected to one ofthe two conductors 5 or 6 of the power line, through at least twodiodes. In FIG. 3, this condition is fulfilled, since Vss is connectedto conductor 5, through the two lower diodes of the diode bridgerectifier 31.

It is clear for persons skilled in the art, that an alternativecomparator with positive and negative supply voltage can be used insteadof the scheme of FIG. 3 and 4. This is however less practical because ofthe necessity of a dual supply voltage and because digital circuits aremostly interfaced with a zero to positive supply voltage.

The diode rectifier bridge 31 can be part of an active rectifier circuitas currently often used in electronic ballast circuits, but this is notan absolute requirement.

The output of the comparator 34 is connected to a microcontroller 35 bythe connection 37. The comparator 34 and the microcontroller 35 aresupplied by a DC supply voltage (VDD), for example a 5 V source,referred to the same reference potential (Vss) mentioned already.

The output of the comparator is a block wave, wherein the lengths of theblocks are in direct relation to the information that is sent. In theexample of FIG. 1, the length of a ‘high’ block at the comparator'soutput is increased by the amount of one delay. The length of said blockis measured by the microcontroller 35. The detected delays can betranslated into digital data, according to the methods described above.For the purpose of measuring the block lengths, the microcontroller 35is preferably equipped with a suitable counter circuit.

In turn, the microcontroller 35 commands the load 2, based on thecontrol signals received, through the connection 38.

In the embodiment of FIG. 3, the receiver is able to send information tothe transmitter, for example in answer to a question asked by thetransmitter on the condition of the load, e.g. is a load defect or not?For this purpose, the transistor 36 may be fired by the microcontrollerthrough connection 39. The presence of diodes D5 and D6 respectivelyconnected to the power line conductors 5 and 6, and of the resistor R6causes an increase of the line current during two consecutive halfcycles as soon as the transistor 36 conducts, R6 being connected betweenthe collector of the transistor 36 and the common cathode of the diodesD5 and D6. One diode (D5 or D6) would be enough to obtain a currentincrease during one half cycle.

The emitter of the transistor 36 is connected to the reference potentialVss. The current increase is preferably limited in time to twohalf-cycles. This increase of line current is detected by thetransmitter, via the shunt resistor 20 and connections 21 22, as statedabove.

A problem with triacs connected to electronic lamp ballasts with activerectifiers occurs when the lamps are switched off. In this case theinput EMI filter draws very irregular input current and this causesirregular switch off of the triac, making it difficult to send data.Also, the reactiveness of the loads is no longer defined and may becomecapacative or inductive to such an extent that sending data becomesimpossible, as explained above. These problems are avoided by keepingpart of the lamps on during communication.

In certain applications, such as domestic applications for example, itis undesirable to leave lamps or loads on at certain moments. To avoidthis, a ‘dummy’ load may be added to the power line, see FIG. 4. Thisdummy load essentially consist of a resistor R7 that can be suppliedthrough the power line by switching on a transistor 40. Such a dummyload may then be commanded by a receiver according to the invention, butsimplified compared to the one in FIG. 3, in that it comprises only thecomparator circuitry to receive command signals from the transmitter.The microcontroller 35 then commands the operation of the transistor 40and therefore the on/off operation of the dummy load, through connection38. The transmitter will keep this dummy load activated as long as noother real load 2 is on, and switch the dummy load off, as soon as atleast one real load is on. Preferably however, the dummy load is notactive all the time, but only during the interruptions of the linevoltage. For example, when the lamps are off, the general reactiveimpedance of the loads may be too high, so that a suitable voltageinterruption cannot be introduced, in order for example to send a‘switch on’ command. If the dummy load is activated, this ensures thatthe phase difference between voltage and current is temporarily restoredto a very low value, so that the ‘hack’ can be sent without problems.After the interruption, the dummy load is again deactivated.

In another embodiment, the function of the dummy load is performed bythe resistor R6 in the receiver circuit, while the switching on or offof the dummy load is done by transistor 36. In this way, a receiveraccording to FIG. 3 may perform all functions of receiving data, sendingback information, AND acting as a dummy load.

According to a preferred embodiment of the transmitter, a dummy load isoperated by the transmitter, and a dummy load circuit may even beincorporated into the transmitter. FIG. 5 shows a dummy load circuitwhich is activated by the microcontroller 12 of the transmitter, throughthe optocoupler 51. The dummy load's circuit comprises a diode bridge(53 to 56) and a resistor RD. In the drawing in FIG. 5, three possiblealternative locations of the resistor RD are shown. It is to beunderstood that only one resistor RD is present in the circuit. Theresistor is actually added to the load impedance seen by the linevoltage between conductors 5 and 6, when the dummy load is activated.Activation of the dummy load of FIG. 5 is preferably also only doneduring the line voltage interruptions. The rest of the circuitry of thedummy load circuit include a diode 57, two resistors in series 58 and59, a Zener diode 61, a capacitor 62 and a resistor 60. The elements 57to 62 are part of a circuit for supplying power to the transistor 52.Any other circuit performing the same function may be used. A resistor63 is present between the microcontroller 12 and the optocoupler 51.

The method and apparatuses of the invention are easy to implement. Themethod and the devices described comply with all internationalstandards. The transmitting and receiving devices require a low powersupply rating. No repeaters are necessary. The short interruptions allowto limit the harmonic current distortions, while the operation at lowfrequencies is beneficial for the immunity to network noise.

1. A device for sending data on an AC power line connected to a powerline voltage, and which is transporting a power line current through aset of two conductors, said device comprising: a switching device,placed in one of the conductors of said power line, a microcontroller, afirst connection between said microcontroller and said switching device,one or more conductors forming a second connection between saidmicrocontroller and at least one of said conductors of said power line,wherein a dummy load circuit is added to the power line, and whereinsaid microcontroller comprises circuitry for producing a signal forchanging a condition of said switching device through said firstconnection, circuitry for measuring an electric value through saidsecond connection in order to detect a zero crossing of said electricvalue, and circuitry for introducing a predefined time delay beforechanging the condition of said switching device, thereby producing aninterruption of the power line voltage, wherein the device for sendingdata further comprises means for activating the dummy load circuitduring said interruption, and deactivating said dummy load after theinterruption.
 2. The device according to claim 1, wherein said dummyload circuit comprises a transistor, a diode bridge, and a resistor. 3.The device according to claim 1, wherein said means for activating saiddummy load circuit comprise an optocoupler.
 4. The device according toclaim 1, wherein said dummy load circuit is incorporated into saiddevice for sending data.
 5. The device according to claim 1, whereinsaid switching device is a triac or a group of two thyristors, saidtriac or said two thyristors being equipped with a snubber network. 6.The device according to claim 1, wherein said switching device is atransistor, and wherein said device for sending data further comprises adiode bridge.
 7. The device according to claim 1, further comprisingmeans for measuring the line current.
 8. The device according to claim7, wherein said means for measuring the line current comprise a shuntresistor in one of the conductors and two connections between saidmicrocontroller and points respectively before and after said shuntresistor.